Some electronic locks have devices contained therein which are controllable electrically and electromagnetic in nature which enable an external force applied through a lever or knob to be connected to the bolt for purposes of retracting the bolt and causing the lock to open. One such lock is described in U.S. patent application Ser. No. 719,046, filed Jun. 21, 1991, entitled "Electronic Combination Lock With High Security Feature", by Gerald L. Dawson et al. Because locks of this type have electromagnetic enabling devices included in their design, and magnetic flux fields can penetrate lock materials to permeate the inner cavity of the lock housing, very strong magnets may be used to attack the lock. The magnetic field can cause the electromagnetic enabling device to operate and render the lock capable of unauthorized operation.
If such a strong magnet is used to attack the lock, the magnetic flux from the magnet may act upon the movable portion of the electromagnetic device, such as an armature of a solenoid or stepper motor, to cause the device to act as if it had been electronically operated by a signal generated by the electronic controls of the lock upon receipt of an authorized combination. An example of this would be the displacement of an armature in a solenoid or the rotation of the armature of a stepper motor Both of the armatures are affected by magnetism which is normally generated by an electrical current passing through a coil in proximity to a displaceable armature member. The displacement of the armature may be accomplished by an external magnetic field if the field is of sufficient strength and proper polarity.
It is a well known technique to position shielding around devices which may be adversely affected by magnetic flux. The typical shielding is an enclosure of steel or other magnetically permeable material which will provide a desired or least resistance path for the magnetic flux and concentrate the magnetic flux in the shield structure, thereby reducing flux densities in adjacent regions. By reducing the flux density surrounding the shield, magnetic manipulations of mechanisms or parts may be reduced or prevented. However, with a sufficiently strong magnetic field, shielding may be helpful but may not be totally effective. High flux densities surrounding very strong magnets can provide a magnetic field which can overcome the shielding effects in a device such as a lock mechanism.
Since the shielding of elements within the lock mechanism may not be totally effective when attacked by a very strong magnet, it then becomes imperative to prevent the lock bolt from being withdrawn and the lock opened should the lock be attacked by a strong magnet. Any shielding which has been placed about the elements of the lock mechanism, such as a stepper motor or solenoid, may fail and not prevent operation of the lock by the attack magnet. The security of the lock under magnetic attack must be assured by other means It is to this end that the subject invention is addressed in order to prevent the displacement of the bolt of the lock from an extended position to a retracted position.
Locks which utilize an electromagnetic device to enable the lock to function upon receipt of an electrical signal from the electronic controls, such as that described in U.S. Pat. No. 5,061,923, to James C. Miller et al., may be attacked by a strong magnet. Typically, the mode of attack is to place the magnet on the outside of the lock housing thereby shifting the displaceable element of the electromagnetic enabling device to a position where the lock cannot be securely locked and the mechanism can be opened without an authorized combination. To attack the lock, after placing the magnet in its appropriate position on the lock housing, the unauthorized individual waits for the authorized individual to close and "lock" the lock mechanism. The authorized individual when locking the device is unaware that a small but powerful magnet has been positioned on and attached to the mechanism of the lock housing and, accordingly, the authorized individual believes that the lock has been operated to be secure. The flux field of the magnet has prevented the lock mechanism from becoming secure when the container is closed. At a later time, the unauthorized individual returns to open the container without the benefit of an authorized combination by merely manipulating the knob or lever to withdraw the bolt, as if the proper combination was entered and the electromagnetic enabling device actuated by the lock controls.